See Tracker.php

The code in Piwik_Tracker.connectDatabase checks to see if self::$db is null and if not returns it.

The code in Piwik_Tracker.disconnectDatabase calls self::$db->disconnect();
but does not set the $db to null. The code should read like so:
public static function disconnectDatabase()
{
if(isset(self::$db))
{
self::$db->disconnect();
// XXX must set $db to null or
// Piwik_Tracker.connectDatabase
// returns unconnected db
self::$db = null;
}
}

We are reading from logs so reuse visits.